Langmuir 1999, 15, 3823-3830
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Voltammetric and Electroreflectance Study of Thiol-Functionalized Viologen Monolayers on Polycrystalline Gold: Effect of Anion Binding to a Viologen Moiety Takamasa Sagara,* Hidehiro Maeda, Yi Yuan, and Naotoshi Nakashima Department of Applied Chemistry, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan Received January 15, 1999. In Final Form: March 15, 1999 The monolayers of thiol-functionalized viologens, i.e., dihexafluorophosphate salts of N-butyl-N′-(4mercaptobutyl)-4,4′-bipyridinium (C4V2+C4SH) and N-pentyl-N′-(5-mercaptopentyl)-4,4′-bipyridinium (C5V2+C5SH), on a polycrystalline gold electrode were characterized in several aqueous electrolyte solutions using cyclic voltammetry and electroreflectance (ER) spectroscopy. The voltammetric response for the viologen dication/radical cation couple (V2+/V•+) in Na2SO4, K2SO4, KPF6, or KF solution was found to be electrolyte anion dependent. However, considerable differences due to the electrolyte anion or the alkyl chain length in the fractional content of dimer forms in the V•+ state and the average orientation of the V•+ moiety, both estimated from the results of ER spectral measurements, were not observed. A large voltammteric peak width was observed in KF solution. Because the voltammetric measurements suggested a strong and weak binding to the viologen moiety respectively for PF6- and F- ions, the dependence of voltammetric response on the concentration of PF6- was investigated quantitatively for a monolayer of C4V2+C4SH in the mixed solutions of KPF6 and KF at a constant ionic strength. It was found that one PF6ion additionally binds to a viologen moiety upon the oxidation of V•+ to V2+. The ratio of the binding constant to V2+ and V•+ was estimated to be approximately 1.2 × 104 M-1 at an ionic strength of electrolyte of 0.4 M. The absence of considerable dependence of the PF6- binding on the ionic strength was suggested by voltammetric measurements in the solutions containing solely KPF6. The electron-transfer rate constants for the monolayers were estimated to be no less in the order of magnitude than 1 × 104 s-1.
Introduction Thiol and disulfide molecules are well-suited design elements for the construction of highly organized thin films on metal surfaces.1 The aligned orientation of alkanethiol molecules is one of the most attractive features of the self-assembled monolayers (SAMs) of such molecules. The monolayers of various types of alkanethiol derivatives with covalently tethered electroactive groups have been investigated in detail.2-20 In such SAMs, it is expected that the orientation of the electroactive moiety, lateral inter* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-95-843-7271 or +81-95847-6749. Telephone: +81-95-843-7271 or +81-847-1111 ext. 2747. (1) For review: Ulman, A. Introduction to Ultrathin Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (2) Sagara, T.; Kaba, N.; Komatsu, M.; Uchida, M.; Nakashima, N. Electrochim. Acta 1998, 43, 2183. (3) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (4) Kondo, T.; Yanagida, M.; Momura, S.; Ito, T.; Uosaki, K. J. Electroanal. Chem. 1997, 438, 121. (5) Finklea, H. O.; Ravenscroft, M. S. Isr. J. Chem. 1997, 37, 179. (6) Yamada, S.; Koide, Y.; Matsuo, T. J. Electroanal. Chem. 1997, 426, 23. (7) Li, J.-H.; Cheng, G.-J.; Dong, S.-J. Thin Solid Films 1997, 293, 200. (8) Tang, X.-Y.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921. (9) Wang, Y.-Q.; Yu, H.-Z.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Liu, Z.-F. Langmuir 1996, 12, 5466. (10) Yu, H.-Z.; Wang, Y.-Q.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. Langmuir 1996, 12, 2843. (11) Redepenning, J.; Food, J. M. Langmuir 1996, 12, 508. (12) Mo, Y.; Sandifer, M.; Sukenik, C.; Barriga, R. J.; Soriaga, M. P.; Scherson, D. Langmuir 1995, 11, 4626. (13) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (14) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203. (15) Shultz, D. A.; Tew, G. J. Org. Chem. 1994, 59, 6159.
action among the active centers, and microenvironment for them are highly regulated. The structure of the SAM may be governed by a number of factors including alkyl chain length, counterion involved in the redox reaction, and surface density of the thiol. Among the structural features, the orientation of the tethered electroactive group with respect to the electrode surface may affect the electrochemical behavior greatly. The orientation is closely related to the functions of the SAMs, for example, the photoinduced electron-transfer function.4,6,21-23 Therefore, it is quite important to explore the structural factors affecting the orientation as well as to establish in situ a method to evaluate the orientation. To describe such factors, studies on the oriented film structure of a SAM of an alkylthiol-functionalized viologen (viologen-thiol) may be well-suited. As a number of publications have described,2,7,8,24-27 the SAMs of viologen-thiols immobilized on a gold electrode surface are stable and give almost reversible redox responses. A (16) Redepenning, J.; Tunison, H. M.; Finklea, H. O. Langmuir 1993, 9, 1404. (17) Sato, Y.; Uosaki, K. Proc. Electrochem. Soc. 1993, 93-11, 299. (18) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (19) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307. (20) Chidsey, C. E. D. Science 1991, 251, 919. (21) Sagara, T.; Kawamura, H.; Ezoe, K.; Nakashima, N. J. Electroanal. Chem. 1998, 445, 171. (22) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (23) Kondo, T.; Ito, T.; Momura, S.; Uosaki, K. Thin Solid Films 1996, 284-285, 652. (24) Hiley, S. L.; Buttry, D. A. Colloid Surf. A 1994, 84, 129. (25) Tang, X.-Y.; Schneider, T.; Buttry, D. A. Langmuir 1994, 10, 2235. (26) De Long, H. C.; Buttry, D. A. Langmuir 1992, 8, 2491. (27) De Long, H. C.; Buttry, D. A. Langmuir 1990, 6, 1319.
10.1021/la9900472 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/06/1999
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viologen-thiol monolayer-modified electrode is a quite useful functional electrode, for example, as an efficient electron-transfer mediator film or as a base film of electroactive multilayered films. The oxidized form of viologen (V2+) is colorless in a visible wavelength region, while the one-electron-reduced form (V•+) exhibits strong absorption bands around 400 and 500-650 nm. Therefore, the redox reaction of viologen provides a suitable chromophore for spectroelectrochemical studies. In our previous publication, we proposed a method to estimate the molecular orientation of a chromophore confined on an electrode surface by the use of electroreflectance (ER) response to polarized incident light.2 We apply this method also in the present work to describe the structures of viologen-thiol monolayers. The aim of the present work was to describe the film structures of viologen-thiol monolayers on a polycrystalline gold electrode using the results obtained by voltammetric and electroreflectance approaches. We used viologen-thiols with symmetric short alkyl chains, butyl and pentyl. Using short alkyl chains with odd and even numbers of carbon atoms, we aimed to know how the chain length affects the orientation of the viologen moiety. However, an odd-even effect was not explicitly observed with these short-chain viologens. Instead, we found a pronounced effect of counteranions which is understandable on the basis of a binding model. Therefore, the main part of this paper focuses on the effect of the anion (especially, hexafluorophosphate) on the electrochemistry.
Figure 1. Cyclic voltammograms of a Au electrode modified with a monolayer of C5V2+C5SH in 0.1 M Na2SO4 and 0.3 M KPF6 solutions at a sweep rate of 50 mV s-1. use of the cell with a flat window with corrected ξ using eq 1 show no difference from those obtained by the use of the cylindrical cell. The instrumentation used for ER measurements is described elsewhere.28,29 A 300-700 nm polarizer (Sigma Koki, extinction ratio 1/10 000) was used for irradiating p- or s-polarized light. The waveform modulating the electrode potential E is described as
Experimental Section Materials. N-Butyl-N′-(4-mercaptobutyl)-4,4′-bipyridinium dihexafluorophosphate (C4V2+C4SH) was synthesized as described in our previous report.2 The synthetic route to prepare N-pentyl-N′-(5-mercaptopentyl)-4,4′-bipyridinium dihexafluorophosphate (C5V2+C5SH) was the same as that to prepare C4V2+C4SH except for the use of pentyl reactants instead of butyl. It was confirmed by 1H NMR that both viologen-thiols were not disulfide but thiol when used for adsorption. Water was purified through a Milli-Q Plus Ultrapure water system (Millipore Co.). Its resistivity was over 18.0 MΩ cm. All other chemicals were of reagent grade. A polycrystalline gold (Au) disk electrode (geometrical area, 2.01 mm2; purchased from BAS) was used as the working electrode. Procedures and Instruments. The Au electrode was polished to a mirror finish by the subsequent use of 0.1 µm diamond paste and 0.04 µm silica suspension, or 0.3 and 0.05 µm alumina suspensions. These two different combinations of polishing particles exhibited no difference in the results of the measurements. After sonication in water to remove the embedded polishing particles, the Au electrode was rinsed well with water and acetonitrile and then immersed in an acetonitrile solution containing a 1 mM viologen-thiol compound to be adsorbed at 25 °C for a 24 h period. The modified electrode was rinsed well with acetonitrile and water and then placed in the spectroelectrochemical cell filled with a deaerated aqueous base electrolyte solution. Two types of spectroelectrochemical cells made of quartz were used: one has a flat optical window and the other has a cylindrical cell body. When using the cell with a flat window, the incident angle to the electrode surface ξ with respect to the surface normal is different from the incident angle at the outer wall of the window θ. The expression of ξ is given as
ξ ) arcsin(sin θ/n1)
(1)
where n1 is the refractive index of the electrolyte solution. To write eq 1, the refractive index of air was assumed to be unity. When the cylindrical cell was used, ξ could be directly set by arranging the optical setup so that the electrode surface was positioned at the center of the cross-sectional circle of the cell. It was experimentally confirmed that the results obtained by the
E ) Edc + Eac ) Edc + ∆Eac exp(jωt)
(2)
where E is the electrode potential, Edc is the dc potential, Eac is the ac potential, ∆Eac is the ac amplitude, j ) x-1, ω ) 2πf, which is the angular frequency (f is the frequency of the potential modulation), and t is the time. The ER signal is defined as the ac component of the reflectance divided by the time-averaged reflectance and designated as ∆R/R. Both the real part (in-phase component with respect to Eac) of the ER signal and the imaginary part (90° out-of-phase component) were monitored simultaneously during the stepwise scan of the wavelength of the incident light λ. For the reference and counter electrodes, a Ag/AgCl electrode in a saturated KCl solution and a coiled Au wire were used, respectively. All potentials cited in this paper are referenced to this reference electrode. All measurements were made in an argon atmosphere at 23 ( 2 °C.
Results and Discussion I. Voltammetric and Electroreflectance Studies in Na2SO4 and KPF6 Solutions. The voltammetric response of a Au electrode modified with a monolayer of C4V2+C4SH in a 0.1 M Na2SO4 solution was described in our previous report.2 Figure 1 shows cyclic voltammograms (CVs) of a Au electrode modified with a monolayer of C5V2+C5SH in 0.1 M Na2SO4 and 0.3 M KPF6 solutions. This concentration of KPF6 was chosen in order for the ionic strength to be equated to that of the Na2SO4 solution. The voltammograms are profoundly influenced by the electrolyte. In both electrolyte solutions, the peak height of the redox response corresponding to the viologen dication/radical cation (V2+/V•+) couple was proportional to the sweep rate v in the range from 5 to 200 mV s-1 for both anodic and cathodic peaks. This reveals that the redox response is due to the surface-confined viologen in the monolayer. The formal potentials E° ′ obtained as the (28) Sagara, T. Recent Res. Dev. Phys. Chem. 1998, 2, 159. (29) Sagara, T.; Takeuchi, S.; Kumazaki, K.; Nakashima, N. J. Electroanal. Chem. 1995, 396, 525.
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Table 1. Summary of CV and ER Data C4V2+C4SH a
Na2SO4 E° ′/mVd ∆Ep/mVe ∆W1/2/mVf fm φ/deg
-335 29 220 0.23 62 ( 1
C5V2+C5SH
b
KFc
KPF6
-459 ( 6 58 ( 25 85 ( 21 0.34 ( 0.05 60 ( 6
CV Data -288 ( 17 5 >250 ER Data 0.26 65 ( 2
a 0.1 M Na SO solution. b 0.3 M KPF solution. c 0.4 M KF solution. 2 4 6 extrapolating to v ) 0 mV s-1. f At v ) 50 mV s-1.
midpoint between anodic and cathodic peak potentials were -413 ( 7 mV in 0.1 M Na2SO4 and -482 ( 9 mV in 0.3 M KPF6. Note that these values are the averages and deviations of at least four separate experiments. The full-width at half-height of the anodic peak ∆W1/2 also depended on the electrolyte to a great extent so that ∆W1/2 ) 175 ( 21 mV in 0.1 M Na2SO4 and 93 ( 21 mV in 0.3 M KPF6 at v ) 50 mV s-1. The parameters obtained from the voltammetric measurements for both C4V2+C4SH and C5V2+C5SH monolayers in 0.1 M Na2SO4 and 0.3 M KPF6 solutions are tabulated in Table 1. The more negative E° ′ and smaller ∆W1/2 in the 0.3 M KPF6 solution compared to those in the 0.1 M Na2SO4 solution were also recorded for a Au electrode modified with a monolayer of C4V2+C4SH. The peak separation between anodic and cathodic peak potentials ∆Ep was virtually independent of v in the range from 5 to 200 mV s-1 for both viologen-thiols regardless of the electrolyte solution. Therefore, the peak separation is not due to the sluggish kinetics of the electron-transfer process. The origin of this peak separation is in question. The amount of immobilized viologen calculated from the peak charge was in the range of (1.8-2.8) × 10-10 mol cm-2 for the monolayers of C5V2+C5SH in both Na2SO4 and KPF6 solutions. This value corresponds approximately to a monolayer coverage, though it is slightly smaller than that for the monolayer of C4V2+C4SH. A greater value of ∆W1/2 in 0.1 M Na2SO4 than that in 0.3 M KPF6 connotes a greater extent of the distribution of the state of a viologen moiety and/or stronger repulsive interaction among viologen moieties in a Na2SO4 solution. Another possible explanation for the large value of ∆W1/2 in a Na2SO4 solution may be given based on the double-layer effect model put forth by Smith and White.13,30 According to the model, the CV peak appears broad when the counterion cannot readily penetrate into the monolayer and the active center is buried in the monolayer at a high surface density. When the present case is adopted, PF6- ion can readily penetrate into the monolayer, while SO42- ion cannot. This model also predicts that the value of E° ′ should be more positive when the active center is buried deeper with respect to the film/solution interface.30 However, the value of E° ′ in the Na2SO4 solution was more positive for C4V2+C4SH than for C5V2+C5SH. This fact connotes that either the tilt of the alkyl tail is greater for C4V2+C4SH or factors other than the double-layer effect should be considered. The dependence of E° ′ on the electrolyte will be discussed later in detail. It is worth noting that the redox response of the V2+/V•+ couple appeared as a single redox wave in the KPF6 solution even when the initial potential for the potential sweep was set at a more negative potential than E° ′. This shows a sharp contrast to the redox response of surfactant (30) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398.
d
a
Na2SO4
KPF6b
-413 ( 7 44 ( 4 175 ( 21
-482 ( 9 44 ( 11 93 ( 21
0.43 ( 0.0 59 ( 2
0.36 ( 0.19 60 ( 3
vs Ag/AgCl in saturated KCl. e Peak separation obtained by
Figure 2. ER spectra (real part) for viologen-thiol monolayers: (A) C4V2+C4SH monolayer; (B) C5V2+C5SH monolayer. Solid line: in a 0.1 M Na2SO4 solution. Broken line: in a 0.3 M KPF6 solution. For all curves, Edc ) E° ′, f ) 14 Hz, and ∆Eac ) 70.7 mV. Nonpolarized incident light was used at ξ ) 32°.
viologens in the NH4PF6 solution reported by Abraham John et al.31 They observed two well-resolved waves and assigned the less negative one to the V•+ dimer. We also found the presence of V•+ dimer by ER measurement (vide infra), while no difference of the redox potential between the monomer and dimer was observed. Figure 2 represents the ER spectra for the two different viologen-thiol monolayer-modified Au electrodes in 0.1 M Na2SO4 and 0.3 M KPF6 solutions. All of the ER spectra measured at the formal potential exhibited a difference absorption feature. Because the V2+ form is colorless in a λ range longer than 380 nm, the ER spectral curves correspond to the absorption spectra of V•+ superimposed on the ER signal from the Au substrate.2 The monomer and dimer of V•+ exhibit absorption maxima respectively at 603 and 552 nm. The absorption spectrum of V•+ is the sum of the contributions from the monomer and dimer. The fractional content of the monomer fm among V•+ forms (31) Abraham John, A.; Kasahara, H.; Okajima, T.; Tokuda, K.; Ohsaka, T. J. Electroanal. Chem. 1997, 436, 267.
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was calculated using
fm ) (ER603/ER552 - 0.59)/0.98
(3)
as described in our previous paper,2 where ER603 and ER552 are the background-corrected ER signal intensities respectively at 603 and 552 nm. The values of fm obtained are listed in Table 1. In a 0.1 M Na2SO4 solution, the value of fm is greater for C5V2+C5SH, while the difference of fm between two viologen-thiols is not seen in 0.1 M KPF6. The ER spectral profile was independent of ξ in the range of 20-60°. The ER spectral profile was also independent of the polarization type of the incident light. These features indicate that the orientation of the viologen moiety of the V•+ form on the electrode surface can be estimated using almost the same method given in our previous paper.2 Briefly, the measurement and data analysis for the estimation of the orientation were made as follows. The ER responses to p- and s-polarized incident light at 603 nm were measured at various incident angles ξ. The data points were presented as a plot of the p/s ratio of the ER signal as a function of ξ. On the other hand, the p/s ratio was calculated on the basis of an optical model of the interface of interest as a function of ξ for various presumed orientation angles φ. The results of the calculation were added in the plot of the p/s ratio as working curves. φ was defined as the angle of the longitudinal axis of the V•+ moiety with respect to the surface normal of the electrode surface, while the azimuth was assumed to be isotropic. The details of the calculation including the optical model were described elsewhere.2 The comparison of the data points to the working curves enables us to ensure the adequacy for the model and to estimate the value of φ. In the present calculation, n1 ) 1.333, the complex dielectric constant of Au substrate at 603 nm, -9.41 + j(1.19),32 and the distance from the the Au surface to the position of the electric dipole of the V•+ moiety of 0.7 nm were used. In the present analysis, the background correction of the ER signal was not used for the following two reasons: (i) To make the background correction, a full wavelength scan ranging from 400 to 700 nm to obtain an ER spectrum is necessary. However, it takes time to measure 20 spectra to have 10 p/s data points, and then time dependence (such as the drift of the instrument or the decrease in redox response) may affect the results. (ii) The p/s ratios obtained without the background correction were virtually the same as those obtained with the correction. A typical plot of the p/s ratio is shown in Figure 3 for a monolayer of C5V2+C5SH in 0.1 M Na2SO4 and 0.3 M KPF6 solutions. Several of the working curves are also shown. For the calculation, a two-phase model2 was used. The use of a three-phase model gave rise to the difference in estimated values of φ less than 3° from the use of the two-phase model. For both monolayers shown in Figure 3, each set of data points falls in line with one of the working curves, indicating that the model used was, in fact, applicable. The estimated values of φ from Figure 3 are listed in Table 1 together with the values obtained for C4V2+C4SH.33 In these four electrode systems, no difference in the orientation angle of the V•+ moiety was found. We also measured the modulation-frequency dependence of the ER signal to examine the electron-transfer kinetics. Among the four combinations of viologen-thiol and electrolyte solutions, the estimation of the electrontransfer rate constant ks could be carried out only for a (32) Kolb, D. M.; McIntyre, J. D. E. Surf. Sci. 1971, 28, 321.
Figure 3. Plot of the p/s ratio of the ER signal versus incident angle ξ. The working curves were calculated for various orientation angles φ as described in the text. The data points of the p/s ratio are as follows: O, C5V2+C5SH monolayer in a 0.3 M KPF6 solution; 9, C5V2+C5SH monolayer in a 0.1 M Na2SO4 solution. The ER responses to p- and s-polarized incident light were measured at Edc ) E° ′, f ) 14 Hz, ∆Eac ) 70.7 mV, and λ ) 603 nm.
Figure 4. Complex plane plot of the ER signal for a Au electrode modified with a monolayer of C5V2+C5SH in a 0.1 M Na2SO4 solution. Edc ) -416 mV, ∆Eac ) 14.1 mV, and λ ) 603 nm. Numbers on the plot indicate modulation frequencies in hertz.
C5V2+C5SH monolayer-modified Au electrode in a 0.1 M Na2SO4 solution. For the other three, the values of ks were found to be greater than the measurable upper limit. Figure 4 displays the complex plane plot of ER signals at various f for a C5V2+C5SH monolayer-modified Au electrode in a 0.1 M Na2SO4 solution. This plot was analyzed by adopting the reported analytical procedure.28,34,35 The assumptions as the basis of the analytical procedure34 are probably not strictly satisfied in the present system because the CV curves exhibited a nonzero peak separation which cannot be ascribed to the sluggish kinetics. However, the trajectory of the ER signal on the complex plane in Figure 4 is typical for the quasi-reversible redox reaction of a surface-confined species.34 Therefore, the use of the analytical procedure should not represent a serious problem for the present purpose of the estimation (33) Taking a glance in our previous paper,2 one might notice a great deal of difference in the position of working curves between Figure 3 in the present paper and Figure 4 in ref 2. However, there is actually no difference: The abscissa of Figure 3 is ξ, while that of Figure 4 in ref 2 is θ. For the relationship between ξ and θ, see the Experimental Section (eq 1). A slightly (ca. 8°) smaller value of φ than the value given in ref 2 was obtained in the present work for the C4V2+C4SH monolayer in a 0.1 M Na2SO4 solution. Because more points of the p/s ratio were used, measurement was accomplished within a shorter period, and high reproducibility was obtained in the present work, we are convinced of the value in Table 1. (34) Feng, Z.-Q.; Sagara, T.; Niki, K. Anal. Chem. 1995, 67, 3564. (35) Sagara, T. Rev. Polarogr. 1998, 44, 6.
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Figure 5. Schematic picture of the orientation of viologenthiols (1, C4V2+C4SH; 2, C5V2+C5SH) with assumptions given in the text. The tail butyl group for 1 and pentyl group for 2 were omitted for simplicity.
of ks. The modulation frequency, at which the imaginary part of the ER signal is zero, f0 satisfies the relation34,35
2πf0 ) (2ks/ARsCd)1/2
(4)
where A is the electrode area, Cd is the double-layer differential capacitance per unit area, and Rs is the solution resistance. Separate CV and impedance measurements enabled us to obtain Cd ) 22.4 µF cm-2 and Rs ) 218 Ω. Using these values and f0 ≈ 2850 Hz (Figure 4) in eq 4, ks ≈ 1.5 × 104 s-1 was obtained as a rough estimate. This value was consistent with the frequency dependence of the ER response in Figure 4 in the range of modulation frequency of 200-5000 Hz. The above-described results of the voltammetric and electroreflectance measurements may be summarized as follows. Two viologen-thiols differ in alkyl chain length. The effect of the chain length was explicitly observed on E° ′ but not on the orientation. The value of E° ′ was less negative for butyl than pentyl viologen, regardless of the counteranion. The V2+ form may be relatively more stabilized for the viologen-thiol with longer alkyl chains. The absence of the pronounced effect of the counteranion on the value of fm is opposed to the reported results for a self-assembled monolayer of an asymmetric viologen surfactant on a glassy carbon electrode.31 This difference may come from the existence of Au-S anchoring in the case of the viologen-thiols adsorbed on a Au electrode. According to our discussion described in the previous paper,2 the orientation is predicted to depend on the chain length. Assuming that (i) the alkyl chain adopts an alltrans conformation, (ii) the tilt angle of the axis of the alkyl chain with respect to the electrode surface is ca. 30° 36,37 for both viologen-thiols used, and (iii) the twist angle of the axis of the alkyl chain is ca. 0° 37 for both, a great difference in φ should be observed between C4V2+C4SH and C5V2+C5SH monolayers. Figure 5 illustrates a possible model exemplifying such a situation. The value of φ ≈ 65°, which is near to the experimentally obtained one, can be predicted for the C4V2+C4SH monolayer. In the case of the C5V2+C5SH monolayer, φ should be 5°. That is, the viologen moiety of C5V•+C5SH is predicted to be up right oriented. On the other hand, the same values (36) Finklea, H. O. Electroanal. Chem. 1996, 19, 109. (37) Truong, K. D.; Rowntree, P. A. J. Phys. Chem. 1996, 100, 19917.
of φ were obtained experimentally for two viologen-thiols. The discrepancy between the results and the prediction connotes that at least one of the above three assumptions is not fulfilled. In other words, the insertion of a viologen moiety midway between alkyl and ω-mercaptoalkyl chains brought about another factor which dominates the film structure compared to the SAMs of alkanethiols. It is unwarranted to speculate what such a factor is only from the present results, but what is surprising is that orientation is nearly the same irrespective of the considerable effects of chain length and counteranion upon the electrochemistry. In the present discussion, the effect of the packing of the alkyl tails extended beyond the viologens upon the tilt of viologens is not included. It is known that the addition of a short alkyl chain to an azobenzene moiety affects the packing of the azobenzenes in the SAM to a great extent.38 Further studies may be necessary to explore the odd-even effect by using a series of N-methylviologenterminated viologen-thiols with various longer root alkyl chain lengths with the complementary use of IR reflection spectroscopy. The effect of the counteranion on E° ′ has been reported for viologen monolayers immobilized on electrode surfaces. Hiley and Buttry have investigated the dependence of E° ′ of monolayers of viologen-thiols bearing fluorocarbon chains on a Au electrode upon a series of counteranions and found that the value of E° ′ goes to more negative potentials when anions interact more strongly with the viologen moiety.24 The value of E° ′ is more negative when the softness and hydrophobicity of the anions are greater. The present result is in line with such a tendency, and PF6- may bind to a viologen moiety more strongly than SO42-. To understand the anion binding in more depth, the anion exchange was tested in section II. Additionally, the binding behavior was analyzed quantitatively in section III. II. Response of Voltammogram to Anion Exchange. When a Au electrode modified with a monolayer of C4V2+C4SH or C5V2+C5SH was first subjected to the CV measurement in a 0.3 M KPF6 solution and then transferred into a 0.1 M Na2SO4 solution, the CV curve profile was restored to be the same as that obtained in a 0.1 M Na2SO4 solution without the experience of the redox reaction in a KPF6 solution even at the initial potential scan. The subsequent, reversal exchange produced the same phenomenon. The voltammetric curve responded reversibly and repeatedly to the exchange of the electrolyte solutions. The ingress/egress of anions accompanied by the redox reaction of viologen-thiol has already been known.26,27 The present results indicate that the counteranion present initially in the film is easily displaced by another anion enriched in the solution phase in no need of the occurrence of the redox reaction. Because the difference in E° ′ between 0.1 M Na2SO4 and 0.3 M KPF6 solutions is greater for C4V2+C4SH than for C5V2+C5SH (Table 1), following experiments in regard to anion binding were focused on the C4V2+C4SH monolayer. The voltammetric responses in mixed solutions of K2SO4 and KPF6 at [K2SO4] + [KPF6] ) 0.1 M were examined.39 As a result, the value of E° ′ was found to shift monotonically to more negative potentials with an increase in the fraction of KPF6. This indicates the (38) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (39) The voltammetric response for a C4V2+C4SH monolayer-modified Au electrode in a 0.1 M K2SO4 solution was nearly the same as that in a 0.1 M Na2SO4 solution. The formal potential was at -327 mV.
3828 Langmuir, Vol. 15, No. 11, 1999
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We assume that the binding of the counteranion to the viologen moiety totally governs the formal potential of the viologen moiety. For the elemental redox process of the viologen moiety in the monolayer, V2+ + e- a V•+, with the standard redox potential of E1, the Nernst equation can be written as
E ) E1 +
Γ(V2+) RT ln naF Γ(V•+)
(5)
where na is the apparent number of electrons involved in the redox equilibrium, Γ(V2+) and Γ(V•+) are respectively the amounts of V2+ and V•+ in the monolayer with a dimension of moles per unit electrode area, and other parameters and constants have their usual meanings. For the anion binding reactions of Figure 6. Cyclic voltammograms of a Au electrode modified with a monolayer of C4V2+C4SH at a sweep rate of 50 mV s-1. Broken line: the last cycle in a 0.4 M KF solution (20.0 mL) under stirring. Arrow: the point at which a 0.4 M KPF6 solution (4.9 mL) was injected. Solid line: the first cycle after the injection of KPF6 under stirring. Dotted line: the steady-state response without stirring reached after KPF6 injection.
presence of binding equilibrium between the viologen moiety and anions. Among the inorganic anions available, F- possesses the highest hardness, and thus, its binding to the viologen moiety should be very weak. In fact, the experimentally obtained value of E° ′ in a 0.4 M KF solution for a C4V2+C4SH monolayer was -288 ( 17 mV (as an average of six separate experiments), which is much less negative than that in a Na2SO4 solution. Therefore, we used mixed solutions of KF + KPF6 in section III to analyze the binding of PF6- quantitatively. It was revealed that the exchange of F- ion in the monolayer for PF6- ion added in the solution phase is quite rapid. Figure 6 represents the change of the CV curve in response to injection of a KPF6 solution in a KF solution under continuous potential scan at v ) 50 mV s-1. The broken line is the CV curve in a 0.4 M KF solution (20 mL) under stirring. At the end of the last anodic scan (approximately 0.0 V) in a 0.4 M KF solution, 4.9 mL of 0.4 M KPF6 was injected using a syringe into the 0.4 M KF solution. The solid line is the first cycle after the injection. This solid line gives almost the same peak potentials as the dotted line, which represents the finally reached steady-state CV curve. The cathodic peak at -0.3 V observed in a 0.4 M KF solution is not seen even on the first cycle after injection. Therefore, we may conclude that the exchange of F- ion in the monolayer for PF6- ion or the penetration of PF6- ion into the interior of the monolayer can be completed within a few seconds. III. Analysis of the Binding of PF6- Ion to the Viologen Moiety. We have already reported a general strategy of thermodynamic analysis of the ion binding to an electroactive center immobilized on an electrode surface and demonstrated the binding of tetraalkylammonium cations to C60 fullerene.40 A similar approach was also reported previously by several groups.11,19,41 The thermodynamic approach in the present work is principally the same as our previous one, though we should hereby deal with consecutive anion binding to viologen. Therefore, we describe below the present approach briefly. (40) Nakanishi, T.; Murakami, H.; Sagara, T.; Nakashima, N. J. Phys. Chem. B 1999, 103, 304. (41) Fawcett, W. R.; Opallo, M.; Fedurco, M.; Lee, J. W. J. Am. Chem. Soc. 1993, 115, 196.
V2+ + pPF6- a V2+‚‚‚pPF6-
(6)
V•+ + qPF6- a V•+‚‚‚qPF6-
(7)
and
the binding equilibrium constant can be given respectively as
K1 )
[V2+‚‚‚pPF6-]
(8)
[V2+][PF6-]p
and
K2 )
[V•+‚‚‚qPF6-]
(9)
[V•+][PF6-]q
where p and q are the numbers of the bound anion and V2+‚‚‚pPF6- and V•+‚‚‚qPF6- designate ion-pair complexes. To write these equations, the activity of the anion in the monolayer was assumed to be equal to that in the solution phase with the activity coefficient of unity. At the formal potential obtained by voltammetry, the total amount of V2+ should be equal to that of V•+ when the reaction is reversible or quasi-reversible with the transfer coefficient of 0.5. That is,
Γ(V2+) + Γ(V2+‚‚‚pPF6-) ) Γ(V•+) + Γ(V•+‚‚‚qPF6-) (10) Using eqs 5 and 8-10, E° ′ is expressed as
E° ′ ) E1 +
1 + K2[PF6-]q RT ln naF 1 + K [PF -]p 1
(11)
6
When K1[PF6-]p . 1 and K2[PF6-]q . 1, eq 11 can be approximated as
E° ′ ) E1 +
(p - q)RT RT ln(K2/K1) ln [PF6-] (12) naF naF
When plotting E° ′ as a function of ln [PF6-], one can obtain (p - q)/na and K2/K1 respectively from the slope and intercept of the straight line expressed by eq 12. Figure 7 displays typical CV curves for a C4V2+C4SH monolayer-modified Au electrode in mixed electrolyte solutions of xMKF + yMKPF6 where x + y ) 0.4. The sum of the concentration of these two salts was kept constant to make the ionic strength approximately constant in order
Electroreflectance Study of Viologen-Thiol
Figure 7. Typical CV curves for a C4V2+C4SH monolayermodified Au electrode at a sweep rate of 50 mV s-1 in xMKF +yMKPF6 solution where x + y ) 0.4: (a) y ) 0.300; (b) y ) 0.0188; (c) y ) 0.001 79; (d) y ) 0.000 073 2.
Figure 8. Plot of formal potential as a function of log([KPF6]/ M) for a C4V2+C4SH monolayer-modified Au electrode: O, in xMKF + yMKPF6 solution where x + y ) 0.4; 9, in a KPF6 solution containing no KF. The solid line is the least-squares fitted straight line in the range of y g 0.001 for the data points obtained in the mixed electrolyte solutions.
to avoid salt concentration dependent shielding of the electrostatic interactions and thus to minimize the salt concentration dependence of the binding constants. This approach also enables us to avoid the complication due to the change in the liquid junction potential at the interface between the working electrode compartment and the reference electrode compartment.16 The CV curves shift monotonically to less negative potentials with a decrease in y, while the CV curve shape is not greatly affected by the concentration in the range of y from 0.001 to 0.4. When y < 0.001, a significant increase in ∆W1/2 is observed. Figure 8 shows the plot of E° ′ as a function of the logarithm of [KPF6]/M, where E° ′ was equated to the midpoint potential between anodic and cathodic peak potentials. The solid line is the best fit to the data points in the range of [KPF6] g 0.001 M. In this range, the value of ∆W1/2 is within 65-125 mV. The slope and intercept to [KPF6] ) 1.0 M are respectively -51.2 and -497 mV. On the basis of the slope, it turns out that (p - q)/na ) 0.875. It is reasonable to conclude that p - q ) 1, because the charge of the viologen moiety increased by +1 upon oxidation of V•+ to V2+. We assume that E1 is equal to E° ′ at [KPF6] ) 0.0 M and [KF] ) 0.4 M, provided that the binding of F- to the viologen moiety is negligibly weak. Then, K1/K2 is obtained as 1.2 × 104 M-1 using E1 ) -288 mV and na ) 1.14. The difference in E° ′ between 0.4 M KF and 0.4 M KPF6, i.e., 209 mV, is identical to the difference reported by Hiley and Buttry for the monolayers of viologen-thiols bearing fluorocarbon chains on a Au electrode.24
Langmuir, Vol. 15, No. 11, 1999 3829
In the present work, ionic binding did not induce any considerable influence on the ER spectral profile. The complexity due to the presence of monomer and dimer V•+ species was not obviously seen in the present system either. It is worth noting that nearly the same binding equilibrium was observed in the solution containing solely KPF6. Squares in Figure 8 represent the values of E° ′ measured in a KPF6 solution containing no KF. These points are almost in line with the straight line in Figure 8. This fact indicates that the value of K1/K2 is insensitive to the ionic strength. IV. CV and ER Measurements in the Solution Containing Solely KF. In the above-described measurements, it was revealed that the CV peak for the V2+/ V•+ couple at higher KF concentrations is quite broad. To understand this behavior, detailed CV and ER measurements were conducted in a KF solution. A typical CV response in a 0.4 M KF solution has been given in Figure 6 as the broken line. The peak current was proportional to v. The results of the measurements are added in Table 1. The value of ∆W1/2 exceeded 250 mV.42 However, the value of ∆Ep (5 mV, see Table 1) was independent of v in the presented range, indicating that the large value of ∆W1/2 is not due to the sluggish electrontransfer kinetics. In fact, the measurement of the frequency dependence of the ER signal indicated that ks is on the order of magnitude of 104 s-1. Therefore, the large value of ∆W1/2 may be due to the distribution of the state of the viologen moiety in the monolayer. However, it should be pointed out that the tendency to show the larger value of ∆W1/2 at a lower concentration of KF is predictable based on the model proposed by Smith and White.30 This is the case for the buried electroactive center in the SAM without diluent alkanethiol.13,30 The ER spectral profile (data not shown) in a KF solution exhibited no considerable difference from that in a 0.1 M Na2SO4 solution. The estimated orientation angle (φ) from the incident angle dependence of the p/s ratio was 64.5 ( 1.4°, which is not largely different from the values in the other electrolyte solutions (Table 1). These results invoke the necessity of further study of the voltammetric response in a KF solution in order to understand the broad peak in light of the models proposed by Smith and White30 and Rowe and Creager,13 though it is beyond the purpose of the present study. Conclusion The neat monolayers of two thiol-functionalized viologens, C4V2+C4SH and C5V2+C5SH, on a polycrystalline gold electrode were characterized in Na2SO4, K2SO4, KPF6, and KF aqueous electrolyte solutions using CV and electroreflectance spectroscopy. The binding of PF6- ion to the viologen moiety was studied in detail. The voltammetric response for the V2+/V•+ couple was found to be electrolyte anion dependent. The dependence of the formal potential of the counteranion showed an order of PF6- < SO42- < F- in line with the increase in hardness from PF6- to F-, consistent with the previous report by Hiley and Buttry.24 On the other hand, considerable differences due to the electrolyte anion in the fractional content of dimer forms in the V•+ state and the average orientation of the V•+ moiety, both estimated from the results of ER spectral (42) We measured CV curves in the solution containing solely KF at different concentrations. The value of ∆W1/2 exhibited a tendency to increase with decreasing the concentration of KF. For example, ∆W1/2 exceeds 300 mV at [KF] ) 4 mM. On the other hand, dependence of the value of E° ′ on the KF concentration was not apparently observed. This fact ensures that the binding of F- ion to the viologen moiety is quite weak and that it is unnecessary to take into account competitive binding of F- ion in the analysis of PF6- binding.
3830 Langmuir, Vol. 15, No. 11, 1999
measurements, were not observed. Examination of the exchange of anions revealed that the counteranion present initially in the film is easily and rapidly displaced by another anion enriched in the solution phase in no need of the occurrence of the redox reaction. Analysis of the dependence of the formal potential on the concentration of PF6- for a monolayer of C4V2+C4SH based on a binding model allowed us to conclude that PF6- ion readily penetrates into the monolayer and that one PF6- ion additionally binds to the viologen moiety upon the oxidation of V•+ to V2+. The ratio of the binding constant to V2+ and V•+ was estimated to be approximately 1.2 × 104 M-1 at an ionic strength of the electrolyte solution of 0.4 M. It was also suggested that the binding equilibrium is insensitive to the ionic strength in a KPF6 solution. The electron-transfer rate constants for the monolayers were estimated to be no less in the order of magnitude than 1 × 104 s-1, irrespective of both the alkyl chain length of the viologen-thiols and counteranion. Although C5V2+C5SH monolayer exhibited more negative formal potential than C4V2+C4SH monolayer in Na2SO4 and KPF6 solutions, no
Sagara et al.
apparent effect of the alkyl chain length was observed on the film structure and electron-transfer kinetics. One must probably seek such an odd-even effect on the film structures by using a series of N-methylviologen-terminated viologen-thiols with various longer root alkyl chain lengths at a better-defined electrode surface. It remained unanswered why CV curves show nonzero peak separation even at reversible conditions. Finally, we emphasize that the finding that the anion concentration dependent redox reaction is understandable on the basis of our binding model provides one of the fundamental aspects of the electrochemistry of viologens. Acknowledgment. This work was financially supported in part by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture, and Sports of Japan for T.S. The authors gratefully acknowledge Mr. Mikio Uchida for his assistance with viologenthiol synthesis. LA9900472
